1. Field of the Invention
The present invention relates generally to navigational signal receivers and, more particularly, to an adaptive temperature compensation method and apparatus for estimating and compensating the temperature-induced frequency drifts of a local reference oscillator in a Global Positioning System (GPS) receiver.
2. Description of the Related Art
Satellite-based radio navigation systems have become widely adopted in many commercial and military applications. Exemplary systems in operation or development include the NAVigation Satellite Timing and Ranging Global Positioning System (NAVSTAR GPS), the Global'naya Navigatsionnaya Sputnikovaya Sistema (GLONASS), an European satellite navigation system called GALILEO, the wide area augmentation system (WAAS), and the local area augmentation system (LAAS). These systems permit a user with an appropriate direct sequence spread spectrum (DSSS) signal receiver to determine his or her position with respect to the Earth.
As an example, the GPS constellation has 24 operational satellites. These satellites are positioned in six different orbital planes such that at any time a minimum of six and a maximum of eleven satellites are visible to any user on the surface of the Earth, except in the polar region. The satellites operate in near circular 20,200 km (10,900 nm, or about 12,000 miles) orbits at an inclination angle of 55 degrees and with approximately a 12-hour period.
Each satellite contains at least one atomic clock and transmits an accurate time and position signal referenced to the atomic clock. A typical GPS receiver locks on to this signal and extracts the data contained therein. With signals from a sufficient number of satellites, a GPS receiver can calculate its position, velocity, altitude, and time.
The GPS satellites transmit data modulated signals at a nominal frequency of 1575.42 MHz. However, due to the velocity of the satellites and the user, this frequency may drift (increase or decrease) by 5 KHz or more. Thus, before locking onto a signal, the local oscillator in the receiver generally searches for available signals in a broad frequency range (i.e., 1575.42 MHz ±5 KHz or more). The broader the search range is, the longer the search time is, and so is the time-to-first-fix (TTFF) of the receiver.
Another important factor that affects the search range and time is the temperature-induced frequency drift of the local oscillator itself. As the local oscillator is exposed to temperature variations, its reference frequency may deviate from the designated value by several KHz. Each crystal used in the oscillator is unique, so the amount of drift also varies. The range of this temperature-induced frequency drift is therefore uncertain and varies from one oscillator to another and increases the search range, especially in the case of high sensitivity GPS receivers, since more frequency components are searched. Therefore, it is highly desirable to decrease this frequency uncertainty range to reduce the acquisition time of GPS signals.
Many efforts have been made to implement various techniques that compensate the crystal oscillator frequency drift due to temperature change. Generally, the temperature-induced frequency drift in the local oscillator frequency cannot be estimated without the help of a priori established look-up table. As such, most of the existing techniques require factory calibration. For example, the U.S. Pat. No. 5,781,073, issued to Mii and entitled, “TEMPERATURE COMPENSATION METHOD FOR AN OUTPUT FREQUENCY DRIFT OF AN OSCILLATOR” discloses a general purpose temperature compensation apparatus including a memory for storing a frequency compensation lookup table and characteristic factors derived from previous tests. These tests were conducted at factory and include a temperature acceleration test and a burn-in test.
According to Mii, during the temperature acceleration test, hundreds of oscillators are placed within a temperature chamber. As the temperature inside the chamber is slowly increased, corresponding output frequencies in response to the temperature variation are processed by a personal computer (PC) to create the frequency compensation lookup table, which is stored in the memory. The memory is likely a Read-Only memory, since the table is populated once with frequency drift values during the factory calibration process.
The U.S. Pat. No. 5,654,718, issued to Beason et al. and entitled, “GPS RECEIVER DEVICE AND METHOD FOR CALIBRATING A TEMPERATURE UNCOMPENSATED CRYSTAL OSCILLATOR” uses a Read-Only memory as well as a Read-Write memory. The CPU receives temperature information and recalls from the Read-Only memory the specified average crystal frequency drift at this known temperature. The frequency drift values had been previously determined and stored in the permanent Read-Only memory.
Another technique involves the use of polynomial of order higher than 3 in the estimation of the frequency drift as a function of temperature. The polynomial coefficients are pre-computed for each crystal and stored in a Read-Only memory. An example of this approach is disclosed in the U.S. Pat. No. 6,509,870, issued to Matsushita et al. and entitled, “SOFTWARE-COMPENSATED CRYSTAL OSCILLATOR” in which a 9th degree polynomial is employed.
A problem with this approach is that the coefficients are different for each crystal. Thus, a lot of prior polynomial fitting is needed, which is time-consuming and can be expensive. Moreover, this approach is not applicable to temperature compensated crystal oscillators (TCXOs). The TCXOs sometimes exhibit reverse temperature characteristics. As such, an average value cannot be determined.
Clearly, there is a need in the art for a more flexible approach that enables adaptive compensation on the temperature-induced frequency drift of a local reference oscillator, that allows for the estimation of the frequency drift based on statistical methods under certain circumstances, and that allows a receiver to self-calibrate without any additional factory calibration or test set up, thus reducing the cost of the receiver. The present invention addresses this need.